This invention relates to a composite semiconductor light-emitting device on which a plurality of light-emitting regions are formed using compound semiconductor layers. Particularly, this invention relates to a composite semiconductor light-emitting device capable of emitting lights of different wavelengths.
There is known a light-emitting diode (LED) capable of emitting lights of different wavelengths. Such a conventional light-emitting diode includes a first light-emitting layer sandwiched between a p-type compound semiconductor epitaxial layer and an n-type compound semiconductor epitaxial layer, and a second light-emitting layer sandwiched between two compound semiconductor layers (containing impurities) whose conductivity types are the same as one of the p-type compound semiconductor layer and the n-type compound semiconductor layer. The second light-emitting layer emits the light whose wavelength is longer than the light emitted by the first light-emitting layer. Such a conventional light-emitting diode is disclosed in, for example, Japanese Laid-Open Patent Publication No. 2001-7401. The light-emitting diode is known as a compound semiconductor light-emitting element that emits white light, which is suitable for a lighting device used in various kinds of display devices such as a backlight of an LCD display device.
The light-emitting element (disclosed in the above described publication) has a layered structure including a single crystal ZnSe substrate (i.e., a lower light-emitting layer) that emits the light upon self-excitation and a ZnSe-based compound semiconductor layer layered on the substrate. A light-emitting layer (i.e., an intermediate light-emitting layer) composed of ZnSeTe and cladding layers sandwiching the intermediate light-emitting layer are formed on the compound semiconductor layer. Another light-emitting layer (i.e., an uppermost light-emitting layer) composed of ZnSe/ZnCdSe multi-quantum well is formed on the upper cladding layer. The uppermost light-emitting layer emits the light of cyan whose wavelength is 480 nm, the intermediate light-emitting layer emits the light of green whose wavelength is 515 nm, and the ZnSe substrate emits the light of yellow.
However, in the layered structure including the semiconductor layers and the substrate that emit lights of different wavelengths as disclosed in the above described publication, it is necessary to match the lattice constants of the layered semiconductor materials to each other, and therefore the kinds of semiconductor materials that can be used are limited. To be more specific, on the single crystal ZnSe substrate, it is possible to grow the same ZnSe-based compound semiconductor layers, but it is difficult to continuously grow a GaN-based semiconductor layer (capable of emitting blue or green light) and a GaAs-based semiconductor layer (capable of emitting red light) on the crystal substrate in terms of lattice constants.
Further, even if it becomes possible to emit lights of the respective wavelengths, there is a case where the intensity of one of the lights of respective wavelengths is weak. In order to uniformly balance the intensities of the lights of respective wavelengths, the intensities of the whole lights must decrease.